1. Field of the Invention
The present invention relates to a glow plug that is used, for example, to pre-heat a diesel engine.
2. Description of the Related Art
In general, the glow plug incorporates a sheath heater having a sheath tube that is made of heat resisting metal. A heating coil constituted by a resistance heating wire is enclosed together with insulating powder. A main metal shell is joined to the sheath heater. A thread portion formed around the outer surface of the main metal shell is used to join the glow plug to the engine block of a diesel engine such that the heating portion formed at the leading end of the sheath heater is positioned in the combustion chamber. In this case, the temperature rising performance of the heater is usually required to have a so-called quick heating characteristic with which a saturated temperature can quickly be realized in order to improve the starting characteristic of the engine. It might be considered feasible to supply a large electric current to the heating coil in the initial stage of the energization to increase the temperature rising rate. However, excessive rise in the temperature of the coil easily takes place in this case, causing a problem of, for example, a breakage of the coil or melting of the sheath tube, to arise.
For example, in a glow plug disclosed in Unexamined Japanese Patent Publication (kokai) No. 59-60125, the glow plug incorporates a control coil made of a material having a positive temperature coefficient of resistance. The control coil is, in the sheath tube, disposed in series with the heating coil so as to improve the quick heating characteristic and prevent excessive rise in the temperature of the coil. In the glow plug having the above-mentioned structure, the control coil has a low temperature in the initial stage of the energization. Because the electric resistance is low, a relatively large electric current passes through the heating coil, thereby causing the heating coil to be heated quickly. When the temperature of the heating coil is raised, the control coil is heated with the heat of the heating coil so that the electric resistance of the control coil is raised. As a result, the electric current which is supplied to the heating coil is lowered. As a result, the temperature rising characteristic of the heater takes the form that the temperature is quickly raised in the initial stage of the energization. Then, the operation of the control coil reduces the supplied current so that the temperature is saturated.
In the above-mentioned publication, the following fact is described. Namely, excessive proximity between the heating coil and the control coil causes the resistance of the control coil to rapidly be raised. In this case, a sufficient time for which the large current is supplied to the heating coil in the initial stage of the energization is not permitted. Therefore, a satisfactory quick heating characteristic cannot necessarily be achieved. As a specific means for solving the above-mentioned problem, a structure is disclosed in which a gap having a length of about 1.5 P to 12 P is provided between the heating coil and the control coil on an assumption that the pitches of wound wires are P. In the above-mentioned publication, a gap of 2mm or larger is required to realize the quick heating characteristic if the outer diameter of the sheath tube is 5 mm and the wire pitches P are 0.6 mm.
In the above publication, it is preferable that a temperature rising characteristic for the glow plug is a characteristic indicated such that E is equal to 4.38. That is, the preferred characteristic is such that the temperature is monotonically raised in the initial stage of the energization after which the temperature is saturated without formation of a peak. However, the inventors of the present invention have found that the above-mentioned temperature rising characteristic is not necessarily a preferred characteristic for the glow plug of a diesel engine mounted on a vehicle.
A battery is employed to serve as a power source of the glow plug of a vehicle or the like. In this case, the glow plug is not always applied with a predetermined voltage (for example, 12 V) of the battery. Usually, a superimposed voltage applied from the alternator or the like is added to the voltage of the battery. Therefore, a voltage higher (for example, 14 V or lower) than the voltage of the battery is usually applied. If the raised voltage is applied as described above, the temperature rising characteristic of the heater raises a problem in that the temperature of the heater is raised excessively because the saturated temperature is raised in proportion to the applied voltage level. To prevent this problem, it is preferable that the temperature rising characteristic (hereinafter called an "excess-rise-preventive temperature rising characteristic") is realized in which a peak temperature is realized in the initial stage of energization after which the temperature is saturated at a level lower than the peak temperature. It should be noted that the above publication teaches that this temperature rising characteristic is not a preferred characteristic (the characteristic indicated such that .epsilon.=6.25 shown in FIG. 10 attached to the disclosure).
The contents of Unexamined Japanese Patent publication (kokai) No. 59-60125 will now be rethought with this fact in mind. If the outer diameter of the sheath tube is 5 mm and the wire pitches Pare 0.6 mm, the above-mentioned excess-rise-preventive temperature rising characteristic can be realized only when a large gap not shorter than 5 mm is provided between the coils as can be understood from FIG. 7 attached to the publication. If the gap between the coils is longer than 6 mm which is somewhat longer than the above-mentioned value, the temperature of the heating coil is excessively raised. The facts state that a stable excess-rise-preventive temperature rising characteristic cannot easily be provided for the glow plug by adjusting the gap between the coils.
In view of the above, available glow plugs have a structure as shown in FIG. 9 formed such that a portion of a sheath tube 100 for accommodating a control coil 102 has a diameter larger than that of a portion of the sheath tube 100 for accommodating a heating coil 101 so as to enlarge the thermal capacity of the portion. Thus, rapid rise in the temperature of the control coil 102 is prevented so as to realize the above-mentioned excess-rise-preventive temperature rising characteristic (refer to, for example, Unexamined Japanese Patent Publication (kokai) No. 3-99122).
In recent years, increase in the performance of diesel engines and size reduction of the same have raised a requirement for a small sheath heater for the glow plug. Thus, the outer diameter of the projection of the sheath tube over the main metal shell is reduced to be smaller than 5 mm. If the outer diameter of the portion of the sheath tube 100 for accommodating the enlarged control coil 102 is made to be smaller than 5 mm, the portion for accommodating the heating coil 101 is made to be even smaller than that value. As a result, the mechanical strength of the portion becomes insufficient. When a shock is exerted, the portion easily broken. What is worse, the outer diameter of the heating coil is too small to obtain a satisfactory increased heating performance. Therefore, when a sheath heater having a small diameter is required, it is preferable that the sheath tube has a simple shape for the purpose of preventing the above-mentioned problems. That is, it is preferable that the shape is formed such that the portions for accommodating the heating coil and the control coil have similar outer diameters. The simple shape of the projection of the sheath tube is as well advantageous to widen the design freedom of the engine head on which the glow plug is mounted.
However, in accordance with the contents of the above-mentioned disclosure, it is substantially impossible to use the sheath tube having a simple shape in which the diameter of the portion for accommodating the control coil is not enlarged and which is shown in FIG. 6 to realize a satisfactory excess-rise-preventive temperature rising characteristic. That is, the reduction in the diameter of the sheath tube and realization of the excess-rise-preventive temperature rising characteristic cannot easily be made to be compatible with each other. Therefore, a glow plug incorporating a sheath heater in which the outer diameter of the sheath tube is 5 mm or smaller and which has a representative excess-rise-preventive temperature rising characteristic has not been realized.